There is a great demand for a method for obtaining an image with high quality employing an electrophotographic image formation process.
As a method for obtaining an image with high quality, an attempt is carried out which employing toner with a small particle size.
Toner with a small particle size may lower the fluidity and cause such image defects that a part of an image pattern lacks. Therefore, in order to improve fluidity of the toner with a small particle size, a method for smoothing the toner surface and sphering the toner is carried out.
Generally, it is necessary in an electrophotographic image formation process that when a toner image is transferred from the image carrier to a recording sheet, toner remaining on the image carrier without being transferred to the recording sheet must be removed from the image carrier. As a method to remove any residual toner remaining on the image carrier, there is one in which the end of a cleaning blade made of an elastic material such as urethane is brought into contact with the image carrier. In this method, one end of the cleaning blade is generally arranged to press against the image carrier in a direction counter to the direction of movement of the image carrier.
It is known that when the cleaning blade as described above is employed, a spherical toner with a small particle size slips through the cleaning blade end, resulting in extremely difficult cleaning.
Various explanations have been made hitherto regarding the phenomenon that the spherical toner slips through the cleaning blade end. The general explanation is as follows. The spherical toners having a large area contacting each other and the same particle size, which are collected in the edge portion (nip portion) of the cleaning blade, are difficult to move over each other, and tend to form a closed-packed structure (structure packed without voids). Such spherical toners further have a large area contacting the surface of an image carrier and strong adhesion, and have a force lifting the edge of the cleaning blade as one aggregate. As a result, the spherical toners slip through the cleaning blade end. Simple increase of the contact pressure of the cleaning blade, which is small in the cleaning effect and rather shortens lifetime of the image carrier, is not applied in many cases.
A method has been studied which eliminates toner remaining on the image carrier surface through a cleaning blade, even when the spherical toner above is employed.
Typical methods are as follows.
(A) A method which supplies to the surface of an image carrier a lubricant reducing a coefficient of friction of the image carrier surface.
The method is disclosed in which even if the spherical toner forms a close-packed structure, slipping property of an image carrier surface is increased by reduction of the coefficient of friction of the image carrier surface, which provides the effect that the toner does not slip through the cleaning blade (see for example, Japanese Patent O.P.I. Publication No. 5-188643).
(B) A method in which an irregular-shaped toner prepared according to a pulverizing method is incorporated as a developer in one developing tank containing one color toner in a four-color full color image formation apparatus.
The method is disclosed in which spherical toner is mixed with the irregular-shaped toner at the vicinity of a nip portion, and does not form a close-packed structure, thereby preventing the toner from slipping through the cleaning blade (see for example, Japanese Patent O.P.I. Publication No. 8-254873).
(C) A method which prevents toner from slipping through the cleaning blade employing a mixed powder material of lubricant particulate coated on the end of the cleaning blade and irregular-shaped toner having an average particle size smaller than spherical toner
The method is disclosed in for example, Japanese Patent O.P.I. Publication No. 2000-267536.
Study on the above (A), (B) and (C) has been made, and the results are as follows.
(Problem of Item (A) Above)
The item (A) above has problem in that most of a lubricant proposed as reducing the coefficient of friction of the image carrier surface are likely to absorb moisture under high temperature and high humidity, and the lubricant adhered onto the image carrier surface has an adverse effect on the charging state, resulting in image faults such that an image lacks.
(Problem of Item (B) Above)
The item (B) can be applied to an image carrier bearing an image with plural colors, but not to an image carrier in a tandem color image formation apparatus. The item (B) above has problem in that since at the beginning of image formation, a sufficient amount of irregular-shaped toner does not reach the end of the cleaning blade, a large amount of spherical toners reaching there slip through the cleaning blade according to the mechanism described above.
(Problem of Item (C) Above)
In the item (C) above, a barrier is formed from the irregular-shaped toner. Since the toner forming the barrier and toner to be dammed by the barrier are of the same kind, it is difficult that only the irregular-shaped selectively reaches the end of the cleaning blade. Therefore, the item (C) has problem in that an efficient barrier as described above cannot be formed, and the spherical toner slips through the cleaning blade.
As is described above, a method has not been found yet which effectively cleans residual toner remaining an the image carrier surface after transfer employing a cleaning blade.
In an image formation method forming a patch image, cleaning of toner (hereinafter also referred to as a patch image toner) forming a patch image is a burden, since the amount of toner to be cleaned in the patch image is more than that of residual toners after transfer.
The patch image refers to one which is employed to correct so as to maintain the normal image density. Typically, a 1.5 cm square patch image of each color is formed on a photoreceptor, and transferred to an intermediate transfer member, wherein a reflection density of each color patch image transferred to the intermediate transfer member is measured employing a detect sensor, thereby controlling so as to obtain a normal image density. When the patch image density is low, charging condition or development condition is controlled to increase the density, and when the patch image density is high, charging condition or development condition is controlled to decrease the density, whereby a print image with good quality is obtained.